1. Field of the Invention
The present invention relates to an ion generator and to a thermal electron emitter used for the ion generator.
2. Description of the Related Art
In a semiconductor manufacturing process, a process of implanting ions into a semiconductor wafer has been implemented as a standard process for the purpose of changing conductivity, for the purpose of changing a crystal structure of the semiconductor wafer, and for some other purposes. An apparatus used in this process is generally called an ion implanter.
As an ion source in such an ion implanter, an indirectly heated cathode ion source is known. In the indirectly heated cathode ion source, a filament is heated by a current to generate thermal electrons which heat a cathode. Thereafter, thermal electrons generated by the heated cathode are accelerated within an arc chamber and collided with source gas molecules therein, so that atoms included in the source gas molecules are ionized. Such a cathode for emitting thermal electrons may comprise, for example, a cathode cap heated by the thermal electron emitted from the filament, and a tubular member of which an end is attached to the cathode cap.
For the source gas molecules to be introduced into the arc chamber, a halide such as a fluoride or a chloride may be used, for example. The source gas molecules of the halide generate halogen radicals in an ionization step, and the halogen radicals act with a component constituting the ion source, such as a metal material for an inner wall of the arc chamber, to form a chemical bond. The chemically-bonded metal material is vaporized and then ionized with the source gas molecules and may be deposited as an ionized substance on the inner wall of the arc chamber or on a surface of the tubular member constituting the cathode.
The cathode cap is preferably maintained at a high temperature in order to improve the generation efficiency of thermal electrons, and the tubular member to which the cathode cap is attached desirably has good thermal insulation properties. However, if a metal material is deposited on the surface of the tubular member when an ion source is operated, the thermal insulation properties of the tubular member are degraded, so that it may be unable to maintain the cathode cap at a high temperature. Accordingly, stable generation of ions will be difficult, requiring maintenance of the component with degraded thermal insulation properties, such as cleaning or replacement. Particularly, in the vicinity of the cathode, which is placed in a high temperature state, a halide tends to dissociate and a metal material is likely to deposit, requiring frequent maintenance. As a result, the productivity is deteriorated in a process using the ion implanter.